Feedback control of the proximity to marginal kink mode stability in ITER-like discharges

DIII-D experiments with discharges that match key ITER normalized parameters demonstrate control of the proximity to the n=1 kink mode pressure limit. Kink mode stability is assessed in real-time by analyzing the plasma response to a small, nonaxisymmetric magnetic perturbation. This response is used as an input for neutral beam injection feedback, which modulates the plasma stored energy, and thus proximity to the pressure limit. The feedback dynamics are described by a scalar model including a single time constant and delay term. This model was used to obtain optimized feedback gains that were evaluated in experiments. In offline analysis, fitting the frequency-dependence of the response measurements to a single mode model allows the growth rate of the driven, stable mode to be determined. The growth rate increases with pressure, extrapolating to a marginal point between the predicted ideal MHD no- and ideal-wall pressure limits. This technique has the potential to enable fusion scenario optimization while avoiding stability limits.